This invention relates to high-quality optics either hot-pressed or extruded from particulate ionic fluorides selected from alkaline earth metal fluorides, alkali metal fluorides and rare earth metal fluorides. The finished optic is essentially free of absorption bands in the range from about 1 .mu. (micron) to about 7 .mu., due to impurities commonly exhibiting absorption bands occurring therewithin.
The application of hot-pressing techniques to pure powder ionic salts is a relatively recent development. It was on Jan. 6, 1958 that a sample of MgF.sub.2 was produced "through which silhouettes, lights, etc. could be distinguished." (See "Optical Materials Research" by W. C. Parsons, Applied Optics, Vol. 11, No. 1, January 1972.) It is still difficult to produce material that is transparent to visible radiation, but this overlooks the fact that transmittance to near-ultraviolet, infrared and near-infrared wavelengths is not necessarily correlatable to transparency in the visible. More importantly, it overlooks the fact that, to date, no ionic powder optic, either hot-pressed or extruded, has been produced which is simultaneously free of absorption bands due to CO.sub.2, H.sub.2 O, hydroxyl (OH.sup.-) ion and acid fluorides (thought to be mainly HF.sub.2 .sup.- and homologous ions of a similar nature).
Prior to the present time, depending on the optical requirements of objects produced from a relatively thick hot-pressed slug, or extrudate, it is possible to diminish CO.sub.2 impurity, that is, minimize but not eliminate the absorption band due to CO.sub.2. This reduction in CO.sub.2 impurity is usually effected at the expense of an increase in both hydroxyl ion and acid fluorides. The mechanism by which this apparent `balance` occurs is not known, but is well-documented from experience. As a result, the highest quality optics are manufactured in recognition of this phenomenon, and by effecting a delicate compromise, often at the expense of yield.
As the production of infrared and ultraviolet transmitting windows and other optical elements, formed particularly by hot-pressing powder fluorides of an alkali metal, alkaline earth metal and rare earth metal, for both military and industrial applications, has been gaining in importance over the past several years, increasingly stringent quality specifications are demanded. The alkaline earth metal fluorides such as magnesium fluoride and calcium fluoride are particularly sought after for relatively high transmittance optics in the ultraviolet, visible and near infrared wavelength regions. In addition to the alkaline earth metal fluorides, many lesser known powder ionic fluorides of one or more of the foregoing metals in combination with each other or with metals of other groups of the Periodic Table have analogous uses. The problems of absorption bands in the usable wavelength range of the lesser known ionic fluorides have been, to date, relatively less important and not surprisingly have received less attention. Therefore, hereinafter, in this specification, the invention will be focused by particular reference to hot-pressed magnesium fluoride and calcium fluoride bodies and to an extruded lithium fluoride body, it being recognized that the problems solved by this invention are common, though to different degrees, to other fluorides.
It is well known that hot-pressed magnesium fluoride and calcium fluoride bodies are not uniformly permeable to radiation throughout all regions of their so-called transmission spectra, but exhibit absorption bands which are undesirable in many applications. Absorption bands at about 1.4, 2.2, 2.3, 2.5, 2.6, 2.8, 2.9, 4.2, 5.0, and 6.7 .mu. wavelength, as well as the water absorption bands at 3.0 and 6.1 .mu. among others, are particularly troublesome in the development of high quality infrared wavelength transmitting elements made of the alkaline earth metal fluorides and rare earth metal fluorides.
Absorption bands in hot-pressed MgF.sub.2 were recognized early in the development of optical bodies from hot-pressed powder. For example, a typical transmittance curve for MgF.sub.2 (see "Hot-Pressing Magnesium Fluoride" by Buckner, Hafner and Kreidl, Jour. of the American Ceramic Soc. 45, No. 9, Pg. 435-438, 1962) shows several sharp absorption bands occurred at the frequency usually taken as O-H bending--. A few extraneous absorption bands -- were observed infrequently, one of which was CO.sub.2 absorbing at 4.2 .mu. (id. Bot. of col. 1, and top of col. 2 at pg. 438).
There is still some question as to the precise identification and correlation of all the ions and their various absorption bands. However, the identity of the absorption bands due to water, carbon dioxide and hydroxyl (OH.sup.-) ion, are recognized. Bands due to acid fluorides such as HF.sub.2 .sup.-, H.sub.2 F.sub.3 .sup.-, H.sub.5 F.sub.6 .sup.- and the like, have been generally either unrecognized or confused with other bands possibly because they were unresolved from other bands, particularly the band at 2.77 due to OH.sup.- ion. It is all these impurities, and their absorption bands, with which we are concerned.
With respect to CO.sub.2 impurity, until recently, the absorption of CO.sub.2 was lightly regarded, mainly because it is not significant in relatively thin optical bodies pressed individually, but with relatively thick bodies in excess of about 1.5 cm. thick, CO.sub.2 absorption becomes a serious problem. Even thin sections in the range from about 2 to about 3 mm, cut from these thick bodies show undesirable CO.sub.2 absorption. The elimination of carbon dioxide from anhydrous powder-optic fluorides is difficult because of the stability of the carbon dioxide layer on the surface of the fluoride powder. Thus, carbon dioxide is not fully displaced by carbon tetrachloride or removed by acid vapors such as hydrochloric, sulfur dioxide, hydrogen fluoride or ammonium hydrogen fluoride. It cannot be eliminated by vacuum pumping even at 1 .mu. pressure at 700.degree. C. The carbon dioxide layer on an alkaline earth metal fluoride protects the fluoride against hydrolysis which accounts for low hydroxyl group contamination in the presence of a relatively high level of carbon dioxide, while in the absence of carbon dioxide, hydrolysis proceeds at as low as about 50.degree. C. Though absorption due to a relatively large amount of CO.sub.2 may be reduced by evacuation of the die or by flowing an inert gas into the molding chamber, such conventional attempts to minimize the absorption bands due to CO.sub.2 are ineffective in relatively thick bodies, and generally result in increased absorption bands for hydroxyl ion and for acid fluorides.
With respect to absorption bands due to hydroxyl (OH.sup.-) ion in magnesium fluoride, Eugene C. Letter, in U.S. Pat. No. 3,114,601, proposed to wet a loose magnesium fluoride powder with an aqueous solution of ammonium fluoride, dry the wetted powder at about 125.degree. C., and then place the mixture in an Alundum tube for about 1 hour at a temperature of between 400.degree. and 600.degree. C. The treated material is then placed in a die in which the powder is hot-pressed to form the desired optical body. It is stated that the time of treatment is relatively unimportant and that the temperature may be varied within relatively wide limits. The optimum temperature range is stated to be from about 400.degree. to about 600.degree. C. Alternatively, aqueous concentrated hydrofluoric acid or anhydrous hydrofluoric acid may be used to wet the magnesium fluoride powder. The powder is thereafter dried prior to being hot-pressed. Another embodiment is to flow a reactive gas such as dry fluorine or anhydrous hydrofluoric acid through the magnesium fluoride, following similar procedures with respect to temperature and times.
The problem with the use of ammonium fluoride in a dry powder mix is that, at the temperatures at which hydroxyl ions are supposed to be substituted by fluorine ions, most of the ammonium fluoride has been driven off and therefore is unavailable for the purpose. Moreover the Letter patent is directed to the removal of OH absorption bands and is ineffective for the removal of CO.sub.2 absorption bands so noticeable in thick slugs of hot-press bodies where the CO.sub.2 does not have an opportunity to escape from the hot-pressed mass. In these prior art procedures the main objective is to remove water without permitting hydrolysis such as would produce a magnesium hydroxy-fluoride which is difficult to compact into an optical body without voids large enough to scatter light. These procedures are of little effect with respect to removal of very small amounts of carbon dioxide, and we are aware of no published study by others with respect to their effect on reduction of acid fluorides.
It has not been possible, until the invention set forth herein, to form either a hot-pressed or an extruded optical body of a pressable fluoride powder in which body the absorption bands due to the CO.sub.2, H.sub.2 O, OH.sup.- and acid fluoride groups are essentially eliminated, either simultaneously or independently.